1. Field of the Invention
This invention relates to a process for producing a vinyl chloride polymer. More particularly, it relates to a vinyl chloride polymer production process by which vinyl chloride polymers having less uneven plasticizer absorption and having good anti-initial discoloration properties and thermal stability can be produced stably.
2. Description of the Prior Art
In processes for producing vinyl chloride polymers by polymerizing in an aqueous medium a vinyl chloride polymer or a monomer mixture containing a vinyl chloride polymer, an antioxidant is conventionally added for the purpose of improving anti-initial discoloration and thermal stability of the vinyl chloride polymers to be obtained and also for keeping polymerization conversion constant and making uneven plasticizer absorption less occur. As to the time of its addition, it is added, e.g., before the polymerization is initiated, during polymerization, at the time the internal pressure has dropped at the last stage of polymerization, or before, during or after the recovery of remaining unreacted monomers. Especially because the antioxidant has a polymerization inhibitory action, it is usually added to a polymerization mixture at the last stage of polymerization also for the purpose of terminating the polymerization.
Since, however, antioxidants are usually in the form of powder, it is difficult to press the antioxidant into a polymerization vessel at a constant rate when the inside of the vessel stands pressurized at the last stage of polymerization. Accordingly, proposed are (1) a method in which an antioxidant is pressed into the polymerization vessel by means of a pump after the antioxidant has been dissolved in an organic solvent such as methanol and toluene, and (2) a method in which a powdery antioxidant is dispersed in an aqueous medium by the aid of a suspending agent or an emulsifying agent to form an aqueous antioxidant dispersion, which is then added by means of a pump. As the pump, a plunger pump and a diaphragm pump are described (Japanese Laying-open Publication No. 60-149608).
The method (1) enables its addition at a constant rate and without any problem even when the inside of the vessel stands pressurized. However, in an instance where monomers are polymerized in an aqueous medium, the organic solvent used to dissolve the antioxidant is included in an effluent emitted out of the polymerization vessel after the polymerization, bringing about the problem of an increase in COD in the effluent. Also, the organic solvent used may cause problems that it mixes into the resultant vinyl chloride polymer to cause an odor of the product and an odor at the time of processing it or that it mixes into monomers recovered when unreacted monomers are collected. Also, some antioxidants have a low solubility in organic solvents, where the organic solvents must be used in a large quantity to make the above problems more serious.
The method (2) can solve these problems. However, when the aqueous antioxidant dispersion is pressed into the polymerization vessel by means of a pump such as a plunger pump or a diaphragm pump when the polymerization vessel is held at an internal pressure of 0.3 to 1.2 MPa at the last stage of polymerization, the pump must have a pump-out pressure higher than the polymerization pressure, requiring a great mechanical energy. Because of such a great mechanical energy, particles contained in the aqueous antioxidant dispersion tend to break or aggregate in the pump at its ball check valve and so forth to which the mechanical energy is most applied especially when the dispersion is being fed. As the result, solid matter may accumulate at such portions to hinder the constant-rate feeding and may further block up the interior of the pump, making it impossible to feed the dispersion. The method (2) has such a great problem. This may seriously occur especially when the pump has a high pump-out pressure, and may cause a great difficulty when the antioxidant dispersion is press-added into the polymerization vessel when its inside stands pressurized at the last stage of polymerization.
Accordingly, an object of the present invention is to provide a vinyl chloride polymer production process that enables addition of the aqueous antioxidant dispersion at a constant rate and stably, without causing any block-up of the interior of the pump even when the polymerization vessel is held at a high internal pressure.
To achieve the above object, the present invention provides a process for producing a vinyl chloride polymer, the process comprising the steps of:
charging into a polymerization vessel, vinyl chloride or a monomer mixture containing vinyl chloride, water, and other materials necessary for polymerization; and
polymerizing in an aqueous medium the vinyl chloride or the monomer mixture containing vinyl chloride;
an antioxidant being added to a mixture of materials before polymerization, or to a polymerization reaction mixture during polymerization or after polymerization, the antioxidant being added in the state of an aqueous dispersion and by means of a uniaxial eccentric screw pump.